This invention pertains to active magnetic thrust bearings and more particularly an active magnetic thrust bearing that acts in cooperation with only a single axial side of a rotor, using an efficient permanent magnet bias for linearized and highly amplified control. Compared with prior art active magnetic thrust bearings that use permanent magnet bias on two or more axial surfaces, the invention offers greatly simplified construction and assembly and reduced costs.
Existing designs of active magnetic thrust bearings have suffered from problems including nonlinear control, inefficient force generation, and complex construction with actuation on two or more axial surf of a rotor. Many designs that use efficient force generation from permanent magnet bias, use one Or more thrust disks attached to the rotating object, and a stator that must be assembled to enclose a disk or to be enclosed by multiple disks. This type of construction is undesirable because it requires high tolerances on multiple piece assemblies, is expensive in terms of the multiple precision pieces and their assembly and because of the difficulties in assembly and disassembly of the rotor and bearing system It would be preferable to have an active magnetic thrust bearing that could operate on a single axial side of the rotor, facilitating much simpler and lower cost construction
A prior art single sided active magnetic thrust bearing 30 using an unbiased electromagnet, shown in FIG. 1, includes a rotor or thrust disk 32 attached to a shaft 31, and a stator 39 constructed of a ferromagnetic yoke 33 located adjacent to the disk 32. The yoke 33 has an electromagnetic coil 34 and two ring poles 35 and 36 that form an axial air gap 37 between the stator yoke 33 and the thrust disk 32. Current through the electromagnetic coil 34 produces a controllable magnetic flux 38 that attracts the thrust disk 32 toward the stator 33.
Unfortunately, the force to current response is nonlinear, which makes control of the magnetic bearing 30 difficult. The force generated is also small for the amount of current in the coil 34. Many turns of the coil 34 could be used to create a high intensity of flux 38 with minimal current, however this increases the inductance of the coil 34 and slows the response time, making it unsuitable for use in magnetic levitation bearings, This magnetic bearing 30 also generates a very high unstable tilting moment because a small change in the distance between the poles 35, 36 and the disk 32 causes a large change in the axial force. The magnetic bearing 30 also produces force only in the vertical direction. A positive current or negative current in the coil 34 both cause an upward force. To increase the force generation per control current and to make the response linear, a large bias current can be continuously run through the coil 34. The control current is then superposed on top of that current to provide a controllable force. The problem with this technique is that the magnetic bearing requires constant power consumption, and the unstable tilt moment generated is very large making full levitation systems more difficult with a nonlinear force-to-position response. Establishing a large bias flux through appreciable air gaps also requires a very large bias current and or number of coil turns.
A single sided active magnetic thrust bearing configuration using a permanent magnet in series with an electromagnet for generating bias flux of prior art is shown in FIG. 2. In this design, a permanent magnet is used to create the bias flux for the bearing. The active magnetic thrust bearing 40 is comprised of a thrust disk 42 attached to a shaft 41, and a cooperating ferromagnetic yoke 43 of an electromagnet 52 that is fastened to a fixed stator 51 and closely spaced from the disk 42. An electromagnetic coil 44 in the electromagnet 52, for generation of a control flux 49, is wound between inner and outer annular ring poles 46 and 47 of the yoke 43. A permanent magnet 45 generates a bias flux 50 without requiring electric power to the coil 44. The control and bias flux 49, 50 exit and enter the stator through the ring poles 46 and 47. The spacing between the thrust disk 42 and the poles 46, 47 of the yoke 43 constitute an axial air gap 48 between the fixed yoke 43 and the rotating thrust disc 42.
Although the permanent magnet 45 can generate a high bias flux 50 without requiring power and the flux can be established over larger air gaps 48, this design of magnetic bearing 40 has several deficiencies. The permanent magnet has a very low magnetic permeability, similar to an air gap. Therefore the control flux 49 created by the coil 44 must drive through a much larger effective air gap, so the amount of control flux generated per amount of coil current is significantly reduced. The force efficiency of the magnetic bearing is lower than desired. Also, operation with a control flux opposite in direction to the bias flux for causing a reduction in anal force can be difficult since the coil must work against the permanent magnet.
Other types of active magnetic thrust bearings that have linear response and efficient force generation have been developed. These thrust bearing use permanent magnets to generate a bias flux and electromagnetic coils to generate the control flux, However in these designs, the bearing is designed such that the coil need not drive the control flux through the permanent magnet. The control flux and the bias flux have non-coincident paths, but they share the portions of their paths including the axial air gaps where the fluxes add or subtract for highly amplified force generation. Because the control flux need not pass through the high reluctance permanent magnet, the amount of control flux per coil current is much greater. Several designs using this principal have been developed. Unfortunately, all such designs work by using two axial sides of the rotor and two or more axial surfaces. The control flux provides a highly efficient force response because the control flux adds with the bias flux on one axial side of the rotor and at the same time is subtracted from the bias flux on the opposite side. A reverse in the control current causes a reverse in the direction of the generated force. The problem with these magnetic bearings is that they require a complicated structure in which the stator must axially enclose a single thrust disk or the stator itself is enclosed by two or more disks. The multiple precision pieces are expensive and assembly and disassembly of machines using these bearings is difficult. The stator is essentially locked around the rotor when assembled. This can hinder magnetic bearing implementation in many applications.
Therefore, a need existed has long for a high force, high efficiency magnetic thrust bearing with a simple construction that can act in cooperation with a single axial side of a rotor.
Accordingly, this invention provides an active magnetic thrust bearing that acts in cooperation with only a single axial side of a rotor that is rotatable about an axis of rotation, while also having an efficient permanent magnet bias for linearized and highly amplified control. The active magnetic bearing uses two concentric ring poles that axially face a ferromagnetic axial surface of the rotor, creating two annular axial air gaps. A permanent magnet in the stator drives a bias flux through a first path including one ring pole, its air gap, the rotor, the second air gap and the second ring pole. The permanent magnet also drives flux through a second path in the stator, by-passing the rotor. The second path has a comparable reluctance to that level of flux produced by the permanent magnet.
An electromagnetic coil in the stator is wound coaxially with the axis of rotation. The coil drives a control flux in a circuit including the second path, both ring poles and axial air gaps. The bias and control fluxes are therefore superposed in the axial air gaps for amplified response. The force generated is proportional to the square of the flux density so a small control flux can result in a large change in axial force exerted upon the rotor. The use of the bias flux also makes this response linear. Because of the inclusion of the second path with reluctance comparable to the path including the a)al air gaps, the electromagnetic coil does not have to drive flux through the permanent magnet. A much higher control flux and higher force is generated from a given coil current and number of turns due to the presence of a lower reluctance circuit for the control flux The reluctance of the bias shunt circuit (the 2nd Path) is high enough to prevent short-circuiting the bias flux
In operation, the control flux either increases or decreases the total flux in the axial air gaps while simultaneously having the opposite effect in the second path. Compared with prior art designs using permanent magnet bias and non-coincident control and bias flux paths, the invention does result in lower force generation per coil current and number of turns. This is because the control flux path provided in the invention has a higher reluctance. However, because the invention can be made to operate on only a single side of the rotor, the construction is much simpler. The permanent bias and control flux path allows generation of much higher forces than previous single sided active magnetic thrust bearings and a linear response. In addition, larger magnets and or larger ring pole surface areas can also be used to further increase the force response. Employing a separate bearing on each of the opposite ends of a structure can also double the axial force on the structure. The bearings would be connected such that, as the force in one bearing is increased, the force in the other bearing is decreased. The second path can include a low permeability section to obtain a reluctance comparable to the path including the axial air gaps and rotor. In one embodiment of the invention, the second path has a shunt portion with a reduced cross sectional area. The reluctance would be very low at low levels of magnetic flux, but at the flux level provided by the permanent magnet, the small cross section area saturates and increases the reluctance in the shunt. The benefit of this configuration is simpler construction, allowing pieces to be fit together tightly and without the use of extra non-ferromagnetic spacers. The benefit of a comparable reluctance in the second path as the path that includes the axial air gaps is to prevent shorting of all of the bias flux from the permanent magnet through the second path, which would result in little or no bias flux traveling through the axial air gaps to the rotor and hence a small and nonlinear bearing force response.
In another configuration and embodiment of the invention, the permanent magnet can be attached to the rotor instead of the stator. This is usually less preferable because of the low strength of permanent magnets in combination with stresses generated during rotation, however, it may offer benefits in particular designs. The permanent magnet and the second path are located on the rotor and the coil is located on the stator as before for connection to electrical power.
The active magnetic thrust bearing is well suited for applications that experience anal force primarily in one direction, experience large axial forces, require simple assemble or disassembly (or both) and benefit from low cost simple construction. Applications include flywheel energy storage systems, turbines and pumps. In the application of flywheel Systems, the invention can work well with heavy flywheel systems such as those employing steel flywheels and especially in systems where the flywheel rotates about a substantially vertical axis. The invention allows generation of very large controllable axial forces for axially levitating a flywheel. The thrust bearing also generates a large passive upward force when located on a top-facing surface so that the bearing can be designed to lift the flywheel with very little control current. The magnetic bearing can also work directly on the axial face of a steel flywheel, which affords a large available surface area. Two sets of magnetic trust bearings can be used on opposite ends of the flywheel to double the force response to control current by one bearing decreasing its force when the force is increased at the other bearing. The bias flux creation from the permanent magnet reduces the unstable tilt moment generated from the thrust bearing and helps make a fill levitation magnetic bearing system more stable.
In another embodiment of the invention, the rotor can have matching pole rings that correspond with the stator pole rings. The two sets of rings magnetically tend to align, thereby producing a passive radial centering force. The invention can be employed in full levitation magnetic bearing systems using active radial magnetic bearings, passive radial magnetic bearings or in systems that use mechanical bearings for radial support.